The invention concerns a method for fluidisation of pulp flow in the headbox of a paper machine or such and control equipment used in the fluidisation.
The making of paper of a good quality and a stable production process make high demands on the headbox of the paper machine. In particular, a headbox meeting qualitative and productive requirements is expected to be able to produce a homogenous and trouble-free lip discharge.
Various applications in operation and further refinement processes make high qualitative demands on paper and board products. In practice, these demands concern the structural, physical and visual characteristics of the products. In order to achieve characteristics suitable for each individual purpose the production processes are optimised at each time for a certain working range, which sets limits usually also limiting the quantity of production. Thus, a product of the desired kind can be made only in a narrow working range of the production process.
Due to the restrictions may be the working range it is very difficult to carry out such changes in the process, which aim at increasing the production and at improving the quality of the product. Significant changes usually require long-range research and technological development. Process changes desirable for an increased productivity of the manufacturing process are e.g. new techniques having to do with an increased machine speed and a minimised use of water (increased web formation consistency).
In order to make paper of a good quality efforts are made to prevent various disturbances, such as vortexes and consistency streaks, from escaping from the headbox. Such disturbance may occur e.g. in connection with fluidisation (a strong geometrical change) and in the output ends of the pipes of the turbulence generator (disturbances from pipe walls, such as vortexes and consistency and speed profiles). For this reason,    1) fluidisation with small geometrical steps and    2) a low pipe-specific flow rate have typically been used in the headbox.
It follows from a low flow rate that the average residence time of the fibre pulp in the headbox after fluidisation is too long as regards avoidance of re-flocculation. Thus, the fibre pulp will now discharge from the headbox in the fluidised state required for a good formation. To improve fluidisation, lamellas have in fact been introduced for use in the headbox. These lamellas are mounted in the lip channel and they bring about more friction surface in the channel. However, the most significant fluidisation-promoting effect of the lamellas relates to their tip turbulences. Although these turbulences are advantageous for the fluidisation, they will cause coherent flow structures in the flow, which will weaken slowly, but which can be seen even in the produced paper. In practice, the added friction surface brought about by lamellas and the resulting increased yield of boundary-layer turbulence are not sufficient to fluidise the flow. However, with the aid of friction surfaces in flow channels and with the aid of boundary-layer turbulence it is possible to maintain the strongly fluidised state brought about in the turbulence generator. An incomplete (cautious) fluidisation carried out in many stages leads to a more disadvantageous floc structure than fluidisation carried out successfully in one go and based on a controlled residence time.